1. Field of the Invention
The invention relates to methods of polymerizing rosin.
2. Brief Description of the Prior Art
The term "polymerized rosin" refers to the resinous mixture obtained when rosin is treated under various conditions with strong inorganic acids, organic acids or acidic clays. The mixture comprises non-dimerizable resin acids such as dehydroabietic acid, resin acids containing some unsaturation which do not react and a number of different types of polymerized resin acids including dimerized resin acids. The mixture also contains a minor amount of esters, resin acid anhydrides and non-saponifiable substances. Polymerized rosin may be refined, usually by distillation, to remove some portion of the monomeric resin acids and other substances to produce a mixture containing a higher concentration of polymerized resin acids. This refined mixture always has a higher softening point than unrefined polymerized rosin and it is referred to as "dimerized rosin" in many textbooks.
Much prior art exits that bears upon the conversion of rosin to a more polymeric substance. Among the literature descriptions of prior art methods, U.S. Pat. Nos. 2,136,525; 2,108,928; 2,307,641; and 2,328,681 are examples; also the U.S. Pat. Nos. 2,515,218; 2,251,806; 2,532,120 and 4,105,462.
In spite of the many methods which have been described for polymerizing rosin, there are at least two disadvantages which usually attend all processes for polymerizing rosin. First, all of the catalysts known to be operable within a reasonable process time to obtain acceptable yields of dimer suffer from some combination of the following disadvantages. They are corrosive; they evolve corrosive or poisonous fumes; their salts are toxic; they need to be used at high levels; they leave residuals (often chemically bonded contaminants) in the final product; they require processing of rosin at relatively low temperatures; and they require processing of rosin in the presence of large amounts of inert solvent. In regard to the latter two points, rosin when used in production quantities is usually stored in the molten condition at temperatures around 150.degree. C. To dissolve it in solvents for processing at 30.degree. to 60.degree. C. requires considerable cooling (a time consuming step). The rosin may crystallize in the process and all the solvent must be stripped out later (which results in low kettle yields). The second major disadvantage of prior art processes, particularly those which use sulfuric acid, is the separation of an acidic sludge or emulsion during work-up. Tall oil rosin polymerization is especially prone to develop a large acid sludge layer which can be very troublesome. The problem is mentioned specifically by Sinclair et al. in U.S. Pat. No. 3,663,545.
The present invention is an improvement over the prior art in that it permits one to attain a high content of polymerized rosin rapidly using a convenient manufacturing sequence.
A substantial advantage of the method of the invention resides in the class of acid catalyst used to promote the polymerization. This class of acids avoids the disadvantages of other previously used strong acid catalysts. They are heterogeneous catalysts which dimerize even tall oil rosin without the formation of large quantities of acid sludge. They operate at higher temperatures (in the range of 60.degree. to 180.degree. C. but preferably in the range of 90.degree. C. to 150.degree. C.). They can be used at levels as low as 0.01% of acid based on rosin (a level of 1 to 20% is preferred) and can be used in a fast, low temperature process.
Another advantage associated with use of the acid catalysts used in the method of the invention resides in their stability, ease of recovery and re-usability. They are particularly useful in a continuous process for polymerizing a rosin. These and other advantages will be described more fully hereinafter.